If you write a scene in which characters sing “Happy Birthday,” prepare to write a check:

Its copyright retains an eternal power to provoke incredulity: Really? I have to pay for that? But Warner Music Group, who acquired it in 1988, collects upward of $2 million a year from film and TV fees off the song.

But is the copyright valid? George Washington University law professor Robert Brauneis says no:

“It is almost certainly no longer under copyright,” he concludes in his study, “due to a lack of evidence about who wrote the words; defective copyright notice; and a failure to file a proper renewal application.”

So where’s that $2 million annual windfall still coming from? Insurers, for one: The insurance necessary on film financing often requires that litigation be avoided by paying all permissions fees. And even without that barrier, it’s simply cheaper to pay the bill than it is to fight Warner.

Paul Collins has the full story for Slate.